I. Field of the Invention
The invention relates to a pitch with a high coking value.
II. Description of the Prior Art
There are two basic types of pitchxe2x80x94coal tar based and petroleum based. These materials have many uses. Pitches with a low softening point, on the order of 40xc2x0 C., are blended with water and clay to seal driveways and/or blended with other materials to seal metal surfaces, e.g., pipe coatings. Petroleum pitches with a 240xc2x0 F. softening point are used as binder pitch for clay pigeons and for graphite electrodes for steel manufacturing. Coal tar pitch with a softening point of about 110xc2x0 C. is used for making anodes for aluminum smelting and electric arc furnaces. There are some petroleum pitches (softening points of 250-500xc2x0 F.) which are used to make carbon fibers. Some manufacturers use a special low sulfur A240 pitch to produce their mesophase carbon fibers.
For some of these applications, coking value is not significant. Driveway sealers are not heated to form coke. For some applications, usually those involving the high softening point pitches, coking value and softening point are critical.
As an example of the criticality of coking value, coal tar pitch is widely used in the aluminum industry, but petroleum pitch is not. The primary reason is that coal tar pitch has a significantly higher coking value than a petroleum pitch with a like softening point.
It is possible to change the softening point of any pitch by distilling it, but the coking value/softening point relationship is hard to change for a given feedstock, i.e., coal or petroleum. Many pitch refiners consider the coking value/softening point curve to be fixed.
It is relatively simple, using distillation techniques (conventional or wiped film evaporator), to produce a high coking value pitch, but the softening point goes up in a predictable fashion. It is also possible to blend coal tar pitch and petroleum pitch and achieve roughly additive effects, i.e., the coking value is about what would be expected from blending two different kinds of pitch together. Blending up to 10-15 wt % of one type of pitch in the other is a common practice to xe2x80x9cextendxe2x80x9d the more costly pitch, which is usually the coal tar pitch.
U.S. Pat. No. 5,746,906, McHenry, et. al., May 5, 1998, disclosed a coal tar pitch/petroleum pitch blend and method of making it. A crude coal tar material is selected according to specific properties including QI, specific gravity, water and ash content. A petroleum pitch material is also selected according to its softening point, QI, coking value and sulfur content. The coal tar pitch is distilled to an uncharacteristically high softening point then mixed with a lower softening point petroleum pitch to a desired softening end point. The material retains significant QI and coking value characteristics of pure coal tar pitch particularly for use in Soderberg-type anodes for aluminum smelting as well as electric arc furnace electrodes. PAH emissions, and more specifically B(a)P equivalent emissions, are all reduced by approximately 40%. The examples disclose blending a petroleum pitch with a softening point of 80xc2x0 C. nominal with coal tar pitch having a softening point of 137.5xc2x0 C. (Example 1) and 138.1 to 147.8xc2x0 C. (Example 2). This patent describes a binary mixture of coal tar pitch and a petroleum pitch having a softening point of ≈80xc2x0 C.
For completeness, fractionation and use of cutter stock for pitch production will be briefly reviewed.
Control of coal tar pitch fractionation is described in U.S. Pat. No. 4,066,159.
Control of petroleum pitch fractionation is straightforward distillation.
Blends of, e.g., a petroleum pitch with cutter stock have been made either to adjust the properties of slightly off spec products or to simplify the production of low softening point pitches. A240 pitch can be cut back with #6 fuel oil to yield pitch specialty products having a low softening point. Some physical properties of various commercial petroleum pitch products are summarized in the following section.
In addition to these high softening point materials, blends of A240 and #6 fuel oil have been made to produce specialty products ranging from thick liquids to materials having a softening point of 110xc2x0 C. In addition to direct production of A225 from the unit, A225 may be made by blending A240 pitch with either A170 or slurry oil.
xe2x80x9cA 500xe2x80x9d pitch is a viscous liquid, a blend of 35% A240 pitch and 65% #6 fuel oil. The product has a viscosity specification of 230-300 centipoise at 160xc2x0 F.
xe2x80x9cA 120xe2x80x9d pitch is a blend of 37% A240 pitch and 63% fuel oil. The semi-solid blend has a softening point of about 111xc2x0 F.
Extensive work has been done towards developing special procedures to produce high softening point pitch.
Conoco reports that oxidation of certain mesophase precursors led to a material that could, with heat soaking, be converted into a mesophase material. This is reported in U.S. Pat. No. 4,892,642, Romine, et al., issued Jan. 9, 1990, Process for the Production of Mesophase, and U.S. Pat. No. 4,892,641, Fuet, et al., issued Jan. 9, 1990, Process for the Production of Mesophase Pitch. In each patent, a carbonaceous feedstock substantially free of mesophase pitch is heated at elevated temperatures in the presence of an oxidatively reactive sparging gas. Subsequent heat soaking and heat treatment of the oxidized isotropic carbonaceous feed is reported to have resulted in substantial quantities of mesophase.
In a paper entitled Air-Blowing Reactions of Coal Tar Pitch 1. Properties of Pitch Modified By Air-Blowing (T. Maeda, et al. Ext. Abst. Nineteenth Biennial Conference on Carbon, University Park, Pa., p. 180 (1989)), researchers of Osaka Gas Company Limited report air-blowing of petroleum derived carbonaceous materials to result in isotropic pitches being produced. Air-blowing was reported as a recognized procedure to raise the softening point temperature and coking value of petroleum derived carbonaceous materials. Hence, the procedure is asserted to be applicable and desirable for producing precursor pitch for isotropic general purpose carbon fibers.
U.S. Pat. No. 4,999,099, Ta Wei Fu and Manfred Katz, discloses a process for heating a carbonaceous feedstock at mesophase-forming temperatures while simultaneously passing a sparging gas containing an oxidative component selected from the group consisting of O2, O3, H2O2, formic acid vapor, and/or hydrochloric acid vapor with an inert gas component to produce a mesophase pitch that is reported to be especially suitable for the manufacture of carbon fibers. The process involves partial oxidation and partial removal of volatile components as a result of the sparging gas.
U.S. Pat. No. 4,209,500, Chwastiak, issued Jun. 24, 1980, discloses a process for making high mesophase content pitch in which carbonaceous feed is heated with agitation and a passing of an inert gas through the pitch.
U.S. Pat. No. 3,976,729 and U.S. Pat. No. 4,017,327, both issued to Lewis, et al., involve agitating a carbonaceous starting material while heat treating same. In DE No. 2221707 and DE No. 2357477, patent applications of Koppers Company, Inc., the manufacture of isotropic carbon fibers is disclosed. The starting material for carbon fibers is first oxidized with oxygen and then vacuum distilled to remove non-oxidized lower boiling components.
This voluminous art could be summarized as follows. Any pitch can be made to have any softening point desired, within limits by conventional heat and distillation treatments. Coal tar pitch and petroleum pitch have similar responses to heat and distillation, but coal tar always has a higher coking value than petroleum pitch for the same softening point. Blends of coal tar pitch and petroleum pitch can be made to give a product with intermediate results. It is not possible using currently available technology to improve the coking value of either coal tar pitch or petroleum pitch, at least not while keeping the softening point constant.
We discovered a way to break free from this constraint and increase the coking value of pitch while keeping the softening point constant. We could also keep the coking value constant while reducing the softening point to improve the workability of the material.
A processed pitch having an increased coking value relative to Mettler softening point prepared by distilling or thermally processing an initial pitch having an initial coking value above 50 wt % and an initial softening point to produce a high coking value and high softening point pitch having a coking value of at least 55 wt % and at 10% higher than said initial coking value and an increased softening point, and softening said high coking value pitch with a liquid hydrocarbon cutter oil to produce a processed pitch having an increased coking value relative to said initial coking value.
In another embodiment, the present invention provides a method of making a pitch having a coking value below 60 wt % comprising distilling or thermally processing a pitch or pitch precursor to produce a high coking value pitch having a coking value above 60 wt % and reducing the coking value of said high coking value pitch by blending with a cut-back solvent; a method of making a pitch having a coking value above 55 wt % and a softening point below 130xc2x0 C. comprising distilling or thermally processing a pitch or pitch precursor to produce a high coking value pitch having a coking value of 60 wt % to 80 wt % and a softening point above 130xc2x0 C. and reducing the coking value and softening point of said high coking value pitch by blending with an aromatic cut-back solvent to produce a pitch having a coking value above 55 wt % and a softening point below 130xc2x0 C.; a method of improving the coking value of an oxidized petroleum pitch having an initial coking value of from 35 wt % to 45 wt % and an initial softening point comprising distilling or thermally processing said pitch to produce a processed pitch having an increased coking value and increased softening point and softening said processed pitch with a cutter solvent to produce a product pitch having an increased coking value relative to said initial pitch; a method of producing a pitch product having a target coking value above 50 wt % and a target softening point comprising:
distilling or thermally processing a pitch or pitch precursor to produce an initial pitch product having a coking value above 50 wt %, a softening point above 240xc2x0 F., and above said target softening point, an initial boiling point and a 10% boiling point;
selecting a cutter solvent having a 90% boiling point and an end boiling point and wherein there is a gap between at least one of said 90% boiling point and said end point of said solvent and at least one of said initial boiling point and said 10% boiling point of said pitch;
blending a sufficient amount of said cutter solvent with said initial pitch product to reduce the softening point thereof and produce a product pitch having a coking value of at least 50 wt %, is a method of improving the coking value of an oxidized petroleum pitch having an initial coking value of from 35 wt % to 45 wt % and an initial softening point comprising distilling or thermally processing said pitch to produce a processed pitch having an increased coking value and increased softening point and softening said processed pitch with a cutter solvent to produce a product pitch having an increased coking value relative to said initial pitch.